Positive cloud‐to‐ground lightning in summer thunderstorms

Abstract

This paper describes cloud‐to‐ground lightning that lowers positive charge to earth over mountainous terrain in the northern Rocky Mountain region. A sample of 14 visually confirmed ground flashes provided criteria for identifying positive ground flashes by using only measured electric fields. According to these criteria, 75 positive ground flashes occurred within the 2830 km2 sampling area during three consecutive summer seasons. About 3% of all ground flashes lowered positive charge. The area density of positive ground flashes is about 0.01/km2 per summer season compared to an area density of negative ground flashes of 0.3/km2 per summer season. Positive ground flashes occurred within the sampling area on 16 of 48 thunderstorm days. Daily occurrences ranged from 1 to 11. When lightning could be associated with individual thunderstorms, the positive flashes usually occurred in sequence during the final 30 min of the storm. The 75 positive flashes are very similar. Each has a single R stroke followed by a continuing current interval. The continuing current duration distribution is approximately log normal, with range 5 to 160 ms and mean of about 60 ms. The prestroke intervals, the total period of field change preceding the first R stroke, are similarly distributed over the range 65 to 210 ms with mean of about 130 ms. Leader field change durations, the latter portion of the prestroke interval, range from 40 to 70 ms with mean of about 50 ms. The positive ground flashes occurred over mountainous and forested terrain free of man‐made structures. Available data indicate that the positive ground flashes for this region may, in most instances, be initiated by positive downward leaders following a horizontally elongated out‐of‐cloud path to ground.

Characteristics of Cloud-to-Ground Lightning in Chinese Inland Plateau.

“…Based on our observation, it seems that the stronger thunderstorm is conducive to produce negative CG flashes, while the weaker thunderstorm is conducive to produce positive CG flashes. Usually, The positive CG flashes are produced in developing and dissipating stage of an individual thunderstorm (Fuquay 1982;Orville et al 1983). This indicates that weaker stage of a thunderstorm is conducive to produce positive CG flashes, and is in agreement to our result in some extents.…”

“…(1) Dissipating stage of an individual thunderstorm (Fuquay 1982;Orville et al 1983). (2) Winter thunderstorms (Takeuti et al 1978;Brook et al 1982).…”

Seasonal, Regional, and Storm-Scale Variability of Cloud-to-Ground Lightning Characteristics in Florida

“…The physical properties of 1CG flashes, the complex nature of Florida's convection, and the increased postupgrade NLDN sensitivity (Cummins and Murphy 2009) likely combine to explain the greater number of large multiplicity 1CG reports during the warm season in Florida. Rakov and Uman (2003, p. 222) noted that 1CG discharges can be initiated by branches of extensive cloud discharges, and that 1CG return strokes often are preceded by significant IC discharge activity that typically lasts in excess of 100 (Fuquay 1982) or 200 ms (Rust et al 1981). Additionally, Rakov (2003) documented an upward bipolar flash that appeared to be associated with a branch of large horizontal extent below the cloud.…”

“…This combination of factors previously has been associated with storms that produce many strong 1CG flashes (e.g., Lang and Rutledge 2006). Since strong 1CG flashes generally have a CC component associated with large charge transfer, they are prime candidates for igniting wildfires (e.g., Fuquay et al 1972). Thus, our case study also examines the polarity of CG flashes near natural wildfire ignitions on 13-14 May 2007.…”

Vertical Electric Fields and Field Change Parameters Due to Partly Inclined Lightning Leader Channels

“…The observation of this inclined channel part inside the cloud is not a feature of only negative return strokes. Fuquay [32] has photographed several positive lightning channels, which have extensively long horizontal paths inside the cloud.…”